Numerous catalyst are known for the hydrogenation of compounds containing unsaturated double bonds, catalysts which may be classified into two groups:
(1) Heterogeneous catalysts, generally consisting of a metal such as Ni, Pd, Pt, Ru, etc. optionally deposited on a support such as carbon, silica, alumina, calcium carbonate, etc.; and
(2) homogeneous catalysts such as (a) Ziegler catalysts consisting of a combination of an organic salt of Ni, Co, Fe, Cr, etc. and a reducing agent such as the organoaluminium compounds and the like, and (b) single component organometallic compounds of Ru, Rh, Ti, La, etc.
U.S. Pat. No. 4,501,857 describes a hydrogenation catalyst in which one of the components is a derivative of cyclopentadienyltitanium--necessarily in the presence of organolithium compounds--for the hydrogenation of the olefinic double bonds of the polymers of conjugated dienes.
European Patent application publication Nos. 0460725, 0549063, 0434469, 0544304, 0545844 and 0601953, International application Nos. WO 96/18660 and WO 96/18655, and British patent application No. 2159819 also describe homogeneous titanium-containing hydrogenation catalyst compositions.
International application No. WO 95/25130 discloses a process for selective hydrogenation of unsaturated polymers containing aromatic and olefinic carbon--carbon double bonds, using as catalyst complex of a metallocene compound, comprising zirconium connected to two substituted or unsubstituted cyclopentadienyl or indenyl groups and to two other ligands selected from halogen, lower alkyl or benzyl, and an alumoxane and preferably methylalumoxane, in a molar ratio of zirconium metal to aluminium metal in the catalyst complex of from 50-500.
However, as known from e.g. European patent application publication No. 0584860, known hydrogenation processes still have as a general shortcoming that in block copolymers, comprising at least blocks formed by poly(conjugated diene) in which the constituting monomer was a branched alkadiene of from 5 to 10 carbon atoms and bearing at least one alkyl substituent to one of carbon atoms of the remaining double bonds, could not be completely or substantially hydrogenated under the usual hydrogenation conditions.
More particularly, said shortcoming is the reason why the known catalysts were satisfactory for the selective hydrogenation of poly(styrene)-poly(butadiene) block copolymers, but not for block copolymers comprising poly(isoprene) blocks or blocks wherein isoprene is a comonomer.
It will be appreciated that the same can be stated for other substituted poly(alkadienes) as well as tri- or tetra-substituted olefins.
The shortcomings of these catalysts are not limited to the hydrogenation of polymeric substrates as described above. The shortcomings extend to low-molecular weight substrates containing similarly substituted double bonds as well, as is exemplified by the work of Broene and Buchwald (J.Am. Chem. Soc. 1993, 115, 12569). Furthermore, the work by Marks et al, as disclosed in U.S. Pat. No. 4,668,773 shows that hydrogenation of olefinic bonds with more than two substituents is indeed a very difficult reaction for which no efficient homogeneous catalysts are known.
It will be appreciated that there is a need for homogeneous "all purpose" catalysts which may hydrogenate all types of olefinic substrates, including the poly(conjugated diene) blocks in block copolymers, in an economical way with respect to catalyst costs (by use of low concentrations) and process time (preferably less than one hour for hydrogenation of more than 80% of the original double bonds and preferably more than 95% and more preferably more than 98%).